Maybe that is OF, but is there a way that you can choose one type of block and delete all of those? Because if you have a big track and you have a lot of decorations and you want removes it's hard to delete one per one.

For items (and be careful doing this) click to delete the item from you items menu.
Game will tell you it has to remove occurrences before it can delete the item, click ok.
Then it will ask if you are sure you want to delete the item, select no (or cancel, I don't remember).
Occurrences of the item will be removed from the map.

I think this feature could be added as a sub-feature of the Copy/Paste mode. In addition to "Select All", there could be various types of selection options, including, but not limited to the following:

"Select terrain" (all terrain or specific types, such as all canyon sections in Canyon or all tree overlay sections in Valley)

After a selection is made, a "Delete" button would be available. This feature could be useful for simply finding blocks as well.

By the way, I'm not sure if this is helpful or relevant, but krial57 created an editor plugin that can delete all blocks, all terrain, or everything (including items). That plugin also has "custom selection" option, but I haven't figured out how or whether that option works. This is the original topic where I found the plugin, and here is krial57's GitHub page, as mentioned on the third page of the topic.